1. Field of the invention
The present invention relates to semiconductor optical devices and a manufacturing method thereof, and more particularly to a technique which is effectively applicable to the structure of a p-type semiconductor layer of a semiconductor optical device including a semiconductor laser, and a method for forming the p-type semiconductor layer.
2. Description of the Related Arts
As a method of crystal growth on an InP substrate for a semiconductor laser, a metalorganic vapor phase epitaxy has been mainly used. Zn is used as a p-type dopant, which is produced from DEZn or DMZn, and Si or Se is used as an n-type dopant, which are produced from Si2H6 or H2Se. Although Si and Se used as the n-type dopants hardly diffuse, Zn used as the p-type dopant easily diffuses. Accordingly, for example, when the concentration of Zn of a p-type guiding layer or a p-type buried layer is increased, the laser characteristic is enhanced up to a certain concentration of Zn along with the increase of the concentration of Zn. However, when the concentration of Zn becomes excessively high, Zn diffuses into an undoped active layer and hence, the laser characteristic is sharply deteriorated. FIG. 13 shows a result of an experiment on Zn characteristic by actually preparing laser devices. When the concentration of Zn was increased to 8×1017 cm−3 from an undoped state, a threshold current value was increased by 4.1 mA although the resistance was lowered by 2.8 Ω. Further, as the p-type dopant having a low diffusion constant compared to Zn, C (carbon) is known, which is produced from CBr4, CCl4 or the like. Although C is used as the p-type dopant with respect to InGaAs, InGaAlAs and InAlAS, C becomes the n-type dopant with a low doping concentration with respect to InP and InGaAsP. FIG. 1 is a schematic cross-sectional view of the device structure when C is used. The device structure is formed by successively laminating an n-Inp cladding layer 2, an n-InGaAlAs lower-side guiding layer 3, an undoped InGaAlAs multiple quantum well active layer 4, a C-doped InGaAlAs upper-side guiding layer 5, a Zn-doped InP cladding layer (1×1018 cm−3) 6, a Zn-doped InGaAs contact layer 7 on an n-InP substrate 1. Profiles of C and Zn in the device structure are shown in FIG. 2. It was confirmed that the Zn diffusion from the cladding layer was stopped at the C-doped layer and Zn did not diffuse into the undoped active layer. When C is doped with a high doping concentration, it is necessary to allow C to grow at a low temperature of 600° C. or less and at a low V/III ratio (a flow rate ratio of V group/III group) (for example, see the relationship between the concentration of C and a growth temperature shown in FIG. 3). Thus, the crystallinity of the InGaAlAs upper-side guiding layer 5 is deteriorated. FIG. 4 shows the relationship between the growth temperature of the InGaAlAs upper-side guiding layer 5 and a PL (photo luminescence) intensity of the device structure. As can be understood from FIG. 3 and FIG. 4, to perform doping of C with the C concentration of 1×1018 cm−3 or more, it is necessary to set the growth temperature to 600° C. or less. (see K. Kurihara et. al., “Carbon-doped InAs(Ga)As with low oxygen contamination”, Journal of Crystal Growth, 254, 2003, pages 6 to 13). On the other hand, the PL intensity of the device structure is sharply lowered when the growth temperature of the upper-side guiding layer 5 is set to 600° C. or less and the crystallinity of the InGaAlAs upper-side guiding layer is deteriorated in the low-temperature growth of 600° C. or less. Hence, it is considered that the PL intensity of a light emitting layer is lowered due to the lack of crystallinity.
Further, while the usual InGaAlAs layer is grown in the vicinity of 700° C., the C-doped layer is grown at a temperature of 600° C. or less and hence, the growth interruption becomes necessary when the temperature rises or lowers. Accordingly, for example, the crystallinity at an interface between the undoped InGaAlAs multiple quantum well active layer 4 and the C-doped InGaAlAs upper-side guiding layer 5 is deteriorated by the growth interruption (K. Kurihara et. al., “Phase separation in InAlAs grown by MOVPE with a low growth temperature”, Journal of Crystal Growth, 271, 2004, pages 341 to 347). FIG. 5 is a Transmission Electron Microscope figure which is obtained as a result of study of the deterioration of crystallinity due to the growth interruption. In this study, to clearly observe the influence of the growth interruption, after growing the C-doped InGaAlAs layer at a growth temperature of 535° C., the growth interruption in the hydrogen atmosphere was provided for 5 minutes for temperature elevation and temperature stabilization, and the InGaAlAs layer was grown at a temperature of 690° C. As a result, it was confirmed that a crystalline defect was generated at an interface where the growth interruption was provided.
FIG. 14 shows a result of an experiment on characteristics by actually preparing laser devices. In spite of the fact that the diffusion of Zn from the cladding layer was stopped at the C-doped layer, a threshold current value was increased by 3.2 mA. The above-mentioned results (FIG. 2 to FIG. 5, and FIG. 14) indicate that although the diffusion of Zn to the active layer can be suppressed by the C-doped InGaAlAs layer, the crystallinity in the growth layer right above the active layer and at the growth interface is deteriorated due to the C-doped InGaAlAs layer which is grown at the low temperature and the low V/III ratio and the growth interruption, resulting in an increased threshold current value.